1. Technical Field
The present invention relates to an optical scanning device and an image formation apparatus, and more particularly relates to an optical scanning device which deflects light beams emitted from plural light sources with a deflection element to carry out scanning exposure and an image formation apparatus including the optical scanning device.
2. Related Art
In recent years, multi-color production of a document has progressed and attempts have been made to improve the productivity of color imaging at an image formation apparatus. A color laser printer which uses plural photoreceptors to improve the productivity of color imaging has come onto the market.
In an exposure device which is used in the image formation apparatus that utilizes plural photoreceptors, a system is used in which plural scanning devices corresponding to the respective photoreceptors are arranged in a row. However, in order to reduce size, reduce number of components and further reduce cost, a system in which plural beams are deflected by a single deflector to scan the plural photoreceptors has been proposed.
As a scanning optical system for forming electrostatic latent images on photosensitive surfaces of respective photosensitive drums, there is a system in which polygon mirrors and image focusing optical systems are provided one-to-one for the respective photosensitive drums. However, providing four sets of polygon mirrors and image focusing optical systems is problematic in terms of cost. Therefore, in recent years there has been a scanning optical system in which a single polygon mirror is utilized in common and plural laser beam fluxes are simultaneously scanned therewith, and thereafter the laser beam fluxes are respectively incident at individually corresponding focusing optical systems and are guided to the respective photosensitive drums.
To respectively separately illuminate the plural light beams onto plural scanned surfaces, it is necessary to separate the plural light beams after deflective reflection by the polygon mirror, and for light sources with the same wavelength, spatial separation is necessary. A required spatial separation can be achieved by, for example, causing the light beam to be incident on a deflection surface (a reflection surface) of the polygon mirror from an oblique angle in a slow scanning plane. However, in a scanning optical device of which the optical structure is compact, because light path length for spatial separation is short, the oblique incidence angle on the reflection surface is large. Consequently, problems arise in that a scanning line on the scanned surface curves and image focusing performance deteriorates.